Method of applying an electroconductive film to a vitreous surface



Ocvt. 24, 1961 P. METHOD OF' APPLYING AN ELE FILM TO A VITREOUS SURFACED. PAYNE,

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United States Patent O 3,005,731 METHOD F APPLYlNG AN ELECTRGCONDUC-TIVE FILM T0 A VHTREOUS SURFACE Paul D. Payne, Jr., Chalfont, Pa.,assignor to Philco Corporation, Philadelphia, Pa., a corporation ofPennsylvania n Filed Feb. 11, 1958, Ser. No. '714,467 Claims. (Cl.117-211) This invention relates to methods for producing an electricallyconductive layer on a substrate, and in particular to improved methodsfor producing on vitreous and ceramic bodies thin, electricallyconductive layers which are transparent to radiation of a selectedwavelength.

There are many cases in which it is desired to produce a conductive and/or transparent layer on a given substrate. A number of theseapplications may be found in the electronic and allied industries inwhich it is desired that glass or similar vitreous substance be socoated that the surface thereof is rendered electrically conductive yettransmissive of light. For example, there are many types ofelectrolnminescent devices which require the formation of a sandwichconsisting of an electro-luminescent substance placed between twoelectrodes, at least one of which must be transparent.

Another example of the use of conductive coatings is in the constructionof photo-index type cathode ray tubes used in the reproduction of imagesin color. A tube of this type is described, for example, in U.S. PatentNo. 2,749,449 issued to W. E. Bradley andD. E. Sunstein on June 5, 1956.ln one form of this type of cathode ray tube a set of indexing elementsis disposed in a predetermined spatial relation to certain elements of aiiuorescent screen on which a multi-colored image is reproduced inresponse to the scanning of an electron beam thereupon. When theelectron beam strikes the indexing elements light of a predeterminedwavelength is emitted which passes through a window formed in theenvelope of the tube behind the screen to a photosensitive device whichthereupon produces electrical indexing signals indicative of the beamposition, these signals thereupon being used to effect coordinationbetween the modulation of the beam and its position.

in the past, this window has consisted simply of a portion of the tubesglass funnel, or flared portion, which has not been coated with thecustomary opaque, conductive second anode coating. It has been found,however,

'that leaving a portion of the funnel uncoated permits that portion ofthe glass to acquire spurous electrical charges thereby distorting theelectric field in the funnel. This distortion, in turn, producesdistortion in the raster, and inthe beam spot. It has therefore beenproposed that the window be not only transparent to the indexingradiation but also conductive (as are other adjacent areas) so as not todisturb the desired electric field within the funnel portion of thetube.

Still another example of the use to which conductive coatings may be putin the electronics industry is the depositionvof an annular layer ofconductive material which acts as an attenuator on the internal surfaceof traveling wave tubes.

Hitherto, Vapplication of transparent conductive layers or iilms tovitreous or ceramic surfaces has required the application (usually byspraying 'or vaporous diffusion) of a halide such as stannous chlorideto the vitreous object which has previously been heated to a point inthe neighborhood of the temperature at which it begins to soften, i.e.,Aabout 500 C. in the case of glass. The stannous 'chloride becomesconverted into a transparent conductive ice layer of stannic oxide whichis closely bounded to the substate.

In the manufacture of photo-index Windows for cathode ray tubes forcolor television of the type described the use of this known method hasbeen found extremely inconvenient and time consuming. This inconvenienceresults from the fact that if the faceplate of the tube has already beenjoined to the funnel portion of the tube, an opening of only about 4diameter is available in which to insert the spray gun into the funnel.Even if it were otherwise feasible to insert the spray gun this methodrequires critical handling of the tube since the latter necessarily hasto be heated to a high temperature before spraying begins. Furthermore,if the fluorescent screen has previously been deposited on the innersurface of the faceplate, heating of the Whole tube to a hightemperature such as the softening temperature of glass may damage thestructure or the alignment of the screen itself. Such heating alsorequires that the glass be carefully Vanneale to avoid the production ofundue strains therein.V

0n the other hand, if only the portion of the glass Where the window wasto be located was heated to the required high temperature great carewould be required in the annealing process to equalize the strainsproduced in that and other portions of the glass. In either event, it isexpensive, in production, to heat the glass up to the softeningtemperature of the glass.

Accordingly, it is an object of my invention to provide a new andimproved method of applying an electrically conductive coating to agiven substrate. Y

lt is another object of my invention to provide a new and improvedmethod of applying to a given object a conductive coating which issubstantially transparent to a selected form of radiation.

Still another object of my invention is to provide an improved methodfor producing a conductive and transparent coating on vitreous andceramic bodies.

Another aim of the invention is to provide a simplified method forapplying a conductive and transparent coating to vitreous and ceramicbodies.

A further object of my invention is to provide a cheaper method forproducing conductive and transparent coatings on vitreous and ceramicbodies.

Another object of my invention is to provide a new method for producingin certain types of cathode ray tubes improved photo-indexing Windowswhich do not disturb the electric field and do transmit the selectedindexing radiation. According to my invention a solution of stannouschloride may be applied to the vitreous substrate While the latter is atroom temperature. Thereafter, the temperature of the substrate itself israised to a temperature of at least about 250 C.

In a preferred form of the invention the stannous chlorider is dissolvedin an ester such as n-propyl acetate and applied to the vitreoussubstrate at room temperature. The ester evaporates almost immediatelyleaving stannous chloride crystals on the substrate. The substrate isthen heated at about 415 C. for about l0 or 15 minutes at the least.

In another form ofthe invention the stannous chloride is dissolved in analcohol such as glycerol and applied to thev substrate which is thenheated at about 300 C. for about one half hour or, alternatively atabout 415 C. for 10 or l5 minutes. In this form of the invention, careis talcento keep the stannous chloride solution from drying before themaximum temperature is reached.

FIGURE 1 is block diagram depicting the general method of forming atransparent conductive coating on a vitreous substance in accordancewith the invention;

FIGURE 2 represents one form of the method according to the presentinvention; and

yabout a half hour.

FIGURE 3 represents another form of the method ac cording to myinvention.

Referring to FIG. l, itfis seen that, in essence, my invention comprisesthe application, at room temperature, 'of a stannous chloride-organiccompound solution to the vitreous material to be coated, and thereafterheating the 'vitreousV material to'about 415 C. for at least about lminutes, although, as will be seen below, a lower ltemperature'maybcemployed for a longer time to give a 'result' equivalenty thereto.The'invention will now be Vexplained in several of its 'alternativeforms.

The preferred form of my invention, as shown in FIG. 2, involves thedeposition of a solution of SnClz 'dissolved in an ester such asn-propyl acetate. This solution may be applied by any conventionalmethod, i.e.,

brushing, swabbing, spraying, flowing on, etc. The solu` tion isprepared by grinding, t0 a fine powder, an anhydrous form of SnCIZ,which may be obtained, for example, under the trade name StannochlorNCNSC-56" -as marketed'by the Metal and Thermit Corporation ofRehwayg/New Jersey. First, a concentrated solution is Vmade consistingof 50 grams oi the anhydrous SnClg and 150 ml. n-propyl acetate. Thissolution is then poured into a small screw neck bottle. rIhe bottle cap:isV preferably lined 'with a piece of polyethylene sheet 'material andscrewed tightly onto the neck. This concentrated solution is then rolledfor about 8 hours to make sure that all the stannous chloride isdissolved. The solution is then ltered through Whatman paper #'42 untilclear to produce a stock solution fromrwhich a diluted solution for theprocess may be obtained. I have found that as little as about 1 gram oras much as about 12 grams of anhydrous SnCl to 100 ml. of n-propylacetate will produce acceptable results for the fabrication of windowsfor cathode ray tubes of the type previously described.' Windows withconductivities per square of 5000 ohms toseveral megohms may be achieveddepending on the thickness of the layer and the concentration of thesolution used. Where resistivities outside ,this range maybe required,quantities of SnClg outside the range 1f-l2 grams can correspondingly beemployed.

After the dilute solution has been prepared it is applied, by brushingor swabbing, on the desired appropriate substrate. In the Acase of colortelevision cathode ,ray tubes of the type previously described it ismost convenient to apply the solution to the inside surface of the glassfunnel of the tube by brushing it onto the desired area. The organicsolvent dries almost immediately upon Aapplication leaving the SnClz incontact Ywith* the glass lSubstrate. The temperature of the substrate isthen raised to about 415 C. for lO'or 15 minutes. desired, the glassneed be raised to only about 250 C.-

Or, if

300 C., and that temperature maintained for at least Theiilm thusproduced has approximately the same characteristics as that produced bythe higher temperature process. It should be noted that in neither caseis the glass heated to the temperature at which it begins to soften. Y tY After the tube has beenallowed to cool gradually there will be founddeposited upon the selected area an iridescent `conductive coating,consisting primarily of stannic oxide,

which transmits visible light and ultraviolet light with 'highe'iciency, the percentage of ultraviolet radiation passing through itbeing as high as 80 percent. As a matter of fact, the deposition of thetransparent and electrically -conductive film in accordance with thisprocess produces ka. window which transmits more ultraviolet light thana window consisting merely of uncoated glass because the iilrn reducesspecular reflection of such light.

If anhydrous stannous chloride of the qualityY desig- 'nated by thetrade name Stannochlor is not available, I have achieved equally goodresults by processing stannous-chloride dihydrate, SnCl2.2H2O to obtainit. This `chemical may be procured commercially, under the desigy`nationReagent Grade, Catalogue T-142, from the Fisher Scientific Company. EThis stannous chloride* di- 110 C. for twenty-four hours.

, lumps, and stored in a tightly-closed bottle. This anhydrous stannouschloride is then used to make a solution in the manner previouslydescribed. c t

if coatings of higher resistivity and less eiiicient ultravioletradiation transmission are Vdesired they may be made by applyingstannous chloride solutions of n-butyl acetate which produces iilms thatare somewhat brownish in color. The preparation of the solution and itsmethod of application is otherwise identical to the n-propyl acetateprocess previously described.

In another form of the'invention, as shown in FIG. 3, the conductivecoating is made by applying a solution of stannous chloride in analcohol such as glycerol, ethylene glycol or triethylene glycol. Thissolution is applied to the subtrate at room temperature in the usualmanner and the subtrate is heated up to about 415 C.

In a preferred version of this form of the invention the solvent used isglycerol which has a high boiling point, care being taken to insure thatthe stannous chloride re mains in solution until the .maximumtemperature to be applied is attained. VIthas been found that if thestannous chloride does not remain in solution until the temperature ofthe substrate has been raised sufficiently high so that the glass formsa bond with the stannic oxide, little opaque islands of a relativelyspongy material cloud the film and lessen its conductivity. TheseYislands are formed because glycerol is normally very hygroscopic andthe SnClz hydrolyzes somewhat in the water contained therein formingSnOHCl and Sn(OH),. Best results are obtained if the glycerol is firstdehydrated to the maximum extent possible and maintained in that State.

Other alcohols having somewhat lower boiling points than glycerol butless ainity for water may alternatively be used in this form of theinvention. However, since the stannous chloride is not likely to remainin solution in such lower boiling point solvents until the temperatureof the glass attains its maximum value there is a slightly greaterpossibility that the above-mentioned nonconductive islands of spongymaterial may be formed with a consequent decrease in the conductivityand light transmissivity of the iilxn formed.

While the invention has been described in terms of the application of aconductive lm to glass as a subtrate,

it should be appreciated that it may also be used with otheryitreoussubstances such as porcelain. It is even possible toA use the inventionto Vdeposit conductive films on surfaces of `ceramic bodies.

I claim:

1. The method of forming an adherent electrically conductive film on avitreous body which comprises applying to said body at approximatelyroom temperature a coating solution of substantially anhydrous stannouschloride and a volatile solvent thereof selected from the groupconsisting of glycerol, ethylene glycol, triethylene glycol 'n-propylacetate` and n-butyl acetate and thereafter heating said body to atemperature of at leest 250 C. Y

2. The method according to claim 1 wherein said ternis maintained at thetemperature to which it is heated for at least about l0 minutes.

4. The method according to claim l wherein said object is maintained atthe temperature to which it is heated for at least about one half hour.

-S`. In a method of forming an adherent electrically conductive lm on asurface of a vitreous subtrate, the steps of applying to said surface asolution consisting essentially of substantially anhydrous stannouschloride and a solvent selected from the group consisting of n-propylacetate, n-butyl acetate, glycerol, ethylene glycol and triethyleneglycol while said surface is at a temperature below that at which anadherent electrically conductive lm is formed vthereon by said solution,and thereafter heating said surface to a temperature at least as high assaid temperature at which said adherent electrically conductive ilm isformed thereon.

6. A method -according Ito claim 5 in which said solvent is n-propylacetate.

7. In a method of forming an adherent, substantially transparent,electrically conductive ilrn on `a surface of a glass subtrate, thesteps of applying a solution consisting essentially of substantiallyanhydrous stannous chloride and n-propyl acetate to said surface Whileit is at a ltemperature below that at which an adherent electricallyconductive film is formed thereon by said solution and thereafterheating said surface to a higher temperature between about 250 C. andthe softening temperature of said glass subtrate.

8. A method according to claim 7, Awherein said stannous chloride has aconcentration in said solution of between about 1 and about 12 grams per100 milliliters of said n-propyl acetate.

9. A method according yto claim 8, wherein said heating step includesthe additional step of maintaining said surface at said highertemperature for at least about a half hour.

10. A method according to claim 8, wherein said higher temperature is atleast about 415 C. and said heating step includes the additional step ofmaintaining said surface at said higher temperature for at least about10 minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,559,969 Kennedy July 10, 1951 2,613,302 Gurewitsch Oct. 7, 19522,614,944 Lytle Oct. 21, 1952 2,617,745 Raymond etal. Nov. 11, 19522,772,654 Herkart Dec. 4, 1956 2,839,424 Labino lune 17, 1958

1. THE METHOD OF FORMING AN ADHERENT ELECTRICALLY CONDUCTIVE FILM ON AVITREOUS BODY WHICH COMPRISES APPLYING TO SAID BODY AT APPROXIMATELYROOM TEMPERATURE A COATING SOLUTION OF SUBSTANTIALLY ANHYDROUS STANNOUSCHLORIDE AND A VOLATILE SOLVENT THEREOF SELECTED FROM THE GROUPCONSISTING OF GLYCEROL, ETHYLENE GLYCOL, TRIETHYLENE GLYCOL N-PROPYLACETATE AND N-BUTYL ACETATE AND THEREAFTER HEATING SAID BODY TO ATEMPERATURE OF AT LEAST 250*C.